The historical background of thermoplastics is by now well known. Some of the first successful thermoplastics, e.g. nylon (polyamide), and polyurethane, developed to compete with nylon, remain high volume products. Demand for increased performance has led to significant advances in thermoplastics. High melt temperature, high tensile strength, thermal stability, ready processability, and other chemical and physical properties have been greatly improved, but generally not at the same time. For example, in the 1960s, several promising polyimide polymers were developed. However, their lack of processability left open only uses which could bear high processing cost. Also in the 1960's and 70's, polyether thermoplastics containing sulfone linkages (polysulfones and polyether sulfones) and ketone linkages (polyether ketones) were developed. These polymers exhibited great strength even at elevated temperatures. However, their market share even today is limited due to their high cost and difficulties associated with their preparation. In some polymers with high melting points, increased crystallinity has rendered the polymers brittle, thus decreasing their suitability for many applications.
In addition to polymers with enhanced mechanical properties, polymers with lessened color, superior optical quality, particularly polymers with low birefringence have been desired. Polymethylmethacrylates and other acrylate-type thermoplastics have seen limited use in optical devices such as lenses and light conduits. However, these thermoplastics have low softening points which limits their applicability. Polycarbonates are widely used for optical disks. However, the high birefringence of higher molecular weight polycarbonates has restricted use to lower molecular weight polymers of lessened physical properties.
Polyesters prepared from conformable rings have been made before. For example polyesters containing residues of terephthalic acid and 1,4-cyclohexane dimethanol ((THDM) have been proposed as polyesters for plastic bottles and the like. However, the modest increase in physical properties of these polymers did not offset the high production cost due to the cost of CHDM. Polymers have also been prepared from high molecular weight, bulky bisphenols, and were found to exhibit high T.sub.g. However, these polymers showed little ductility. Their brittleness prevented their use in many optical devices such as optical cables and optical disks.
It would be desirable to provide new polymers with a desirable combination of chemical and physical properties. In particular, it would be desirable to produce thermoplastic polymers which exhibit high melt temperatures and high T.sub.g while remaining ductile rather than brittle. It would be further desirable to provide such thermoplastics which, despite their high softening points, can be made optically clear with low birefringence, encouraging use as optical elements in hostile environments or where increased data storage capability and long term storage stability is desired. It would be yet further desirable to provide high T.sub.g polymers which can be solvent processed as well as being thermally processable.